In Europe and North America, there are currently about 10,000 people on lung-transplant waiting lists. Each year, about 2500 people are transplanted, of whom approximately 2000 survive to live healthy lives. Each year about 2500 die on the waiting list, during a typical 2-year waiting period. The situation is actually far worse than the statistics would indicate because a much larger number of people are never entered onto waiting lists. These people may be excluded because they have no chance of surviving the wait for a transplant or because they are too old. There is little prospect that the situation will improve because the availability of donor organs is declining. The availability of suitable prosthetic lungs would revolutionize the situation. However, to date the development of prosthetic lungs has been deterred because of the perceived difficulty involved in reproducing the structure and function of a human lung. In addition to such deaths from chronic lung conditions, there are millions of deaths from acute conditions. For example, the WHO quotes nearly 4 million deaths per annum for Acute Respiratory Infection. In principle, a large proportion of these people have recoverable conditions and a device that directly oxygenated blood for a limited period would enable these people to recover.
The Applicant has now developed an improved blood/air mass exchange apparatus that is amenable for use as part of an external or intermediate respiratory aid for use by a patient with impaired lung function. Applicant's earlier published PCT Patent Application No. W02005/118025 describes a mass exchange apparatus that functions as a counter-diffusion device to transfer oxygen from the air into the blood and carbon dioxide from the blood to the air. The blood and air flow in alternate channels or conduits. The walls defining the channels or conduits are gas-permeable to allow the required mass transfer. Applicant has now devised various improvements relating to the control of such a mass exchange apparatus. The improvements provide for more stability in use, and hence enhanced patient treatment.
The present application describes blood oxygenation using a membrane mass exchange apparatus that provides stable control of respiration for a conscious mobile patient. It addresses the specific problem that both the oxygen and carbon dioxide concentrations in the blood must be controlled. It recognizes that the body has its own internal control mechanisms based primarily on sensing blood carbon dioxide concentration and that no external controller has access to the natural biological set point. It further recognizes that within a membrane mass exchange apparatus the mass transfer coefficient for carbon dioxide may be an order of magnitude greater than that for oxygen. It consequently addresses the overall problem of providing a stable system suitable for use with a conscious mobile patient.
It is an object of the present invention to provide an improved mass exchange apparatus for use in blood/air mass exchange. It is also an object of the present invention to provide an improved external respiratory aid for use external to a human body. It is a further object of the present invention to provide an improved intermediate respiratory aid for use part internal to a human body and part external thereto.